Slow Cooker BBQ Beef

I like to make this recipe, then shred the meat and serve it on fresh buns for a great BBQ beef sandwich.

1 (3 lb.) boneless sirloin tip roast, cut into large chunks
3 celery stalks, chopped
1 large onion, chopped
1 green bell pepper, chopped
1 c. ketchup
1 can tomato paste
1/2 cup packed brown sugar
1/4 cup cider vinegar
3 T. chili powder
2 T. lemon juice
2 T. molasses
2 tsp. salt
2 tsp. Worcestershire sauce
1 tsp. dry mustard

Place meat and veggies in crock pot. Mix rest of ingredients and pour over the top. Cook on low for 8-9 hours.